Breast cancer is one of the few human malignancies to endocrine manipulation, with the presence of estrogen receptor (ER) in breast tumors identifying a group of patients most likely to respond to endocrine therapy. Unfortunately, though about 60% of breast cancer patients are positive for ER expression, only about 30% respond significantly to endocrine therapy. Additionally, some breast tumors initially estrogen- responsive eventually become resistant to endocrine therapy. Therefore, defining the factors which affect ER expression and function are crucial to understand breast cancer behavior and how to choose effective treatment. Because structural polymorphisms or mutations within domains can directly affect expression and protein function, we here propose to examine the ER sequences for structural alterations of functional significance. With the new technique termed polymerase chain reaction, PCR, it is now possible to rapidly clone and sequence genes. We will address the feasibility of using the PCR amplification technique to amplify, clone, and sequence ER sequences from clinical breast tumor specimens. We will focus on three situations in which there is a discordance between ER and the estrogen- induced progesterone receptor (PR), or between ER and hormone response of the tumor. Using the PCR technique to examine selected portions of the ER sequence, we will ask: 1) whether there are mutations in the ER hormone binding domain sometimes responsible for the ER-PR+ phenotype; 2) whether there are mutations in the ER DNA binding domain sometimes responsible for ER+ PR-phenotype; and 3) whether there are mutations in either domain sometimes responsible for the failure of hormone therapy in ER+ breast cancer. Finally, we will ask: 4) whether in a larger series of clinical breast cancer specimens, these mutations correlate with ER expression and with recurrence and survival. These analyses will give valuable insight into the molecular defects or polymorphisms which impact on either ER expression or hormone- responsiveness during therapy. Therefore, the proposed work could directly affect treatment strategies for breast cancer.